It is understood that the problem of detecting defects is all the more important the longer and more complex the electric cable networks or the more difficult they are to access (buried cables for example). This is why remote detection and location systems have been contemplated, operating from one end of the cable. The procedures used are so-called reflectometry procedures, in which a signal injected at one end of a cable propagates in this cable and a part of the amplitude of the signal is reflected at the site of the defect, because of the impedance discontinuity that the signal encounters at this site. If the speed of propagation of the signals in the cable (related to its characteristic impedance) is known, the measurement of the duration which separates the emitted wave from the reflected wave gives an indication of the distance between the end of the cable and the defect.
In temporal reflectometry procedures (TDR for “Time-Domain Reflectometry”), an electromagnetic wave is injected into the cable in the form of a voltage pulse, a voltage step change, or other. The wave reflected at the site of the impedance discontinuity is detected at the site of the injection and the time gap between the emitted and received edges is measured. The position of the defect is determined on the basis of this gap, and the amplitude and the polarity of the reflected pulse give an indication of the type of defect (open circuit, short-circuit, resistive defect, or other).
There also exist frequency domain reflectometry procedures (FDR for “Frequency Domain Reflectometry”), which consist in injecting at the input of the cable a sinusoid which is frequency-wobulated continuously or by step changes and in measuring the frequency gap or phase gap between the emitted wave and the reflected wave. The published patent application WO 02/068968 describes a frequency domain reflectometry procedure. In a variant called SWR for “Standing Wave Reflectometry”, the nodes and antinodes of a standing wave generated by the combining of an incident wave and its reflection are detected.
Frequency domain reflectometry procedures are effective for analyzing a simple cable. They are difficult to use when the cable comprises branch-offs. Time domain reflectometry procedures can be used even with branch-offs but the analysis of the reflected signals is difficult because of the presence of multiple reflections.
A procedure based on both time and frequency and consisting in injecting a linearly wobulated signal with a Gaussian amplitude envelope has also been proposed, in the published patent application WO 2004/005947.
Spread spectrum reflectometry procedures have also been proposed, in the article “Spread Spectrum Sensors for Critical Fault Location on Live Wire Networks” by Cynthia Furse et al., in Journal of Structural Control and Health Monitoring, Volume 12, Issue 3-4, 2005. A signal is transmitted in the form of a low-level pseudo-random code on a network, even when it is in service; this signal and its echo reflected by a possible defect are correlated with variable time offsets to establish a time-dependent correlation curve. This curve exhibits correlation spikes at time offsets related to the position of the defects and junctions and/or branch-offs of the network. This system is particularly suited to the detection of intermittent defects since it can operate even though the network is being used; however, intermittent defects may very well occur only when the network is in service and disappear when it no longer is (for example a defect which occurs while an airplane is flying but which disappears on the ground). This procedure can be used for cables comprising branch-offs, but it retains ambiguities: it is not possible to say on which branch a detected defect is situated.
Finally, a similar procedure but using quite simply the signals or the natural noise circulating in the cable, and not a pseudo-random code injected at the input of the cable, has been proposed in the article by Chet Lo and Cynthia Furse “Noise-Domain Reflectometry for Locating Wiring Faults” published in IEEE Transactions on Electromagnetic Compatibility, Vol 47 No 1 Febuary 2005. Spikes with strong correlation are detected in a process for correlating the signal with itself. This procedure suffers from the same defect as the previous one, that is to say it does not allow easy resolution of position ambiguities when there are several branches.
The aim of the invention is to resolve these ambiguities, notably in cables exhibiting a T-structure (also called a Y-structure), that is to say comprising at least one branch-off.